At present, as a method for fractionating dextrin, there are mainly a fractionating method by exclusion chromatography and a fractionating method by precipitation with alcohol. The former is costly due to expensive fillers and expensive chromatographic power systems (i.e., pumps). Additionally, by the fractionating method by exclusion chromatography, it is difficult to realize industrial production due to small capacity. The fractionating method by precipitation with alcohol has the following disadvantages: it is likely to have local co-precipitation during the addition of alcohol thus to cause poor fractionation effect; the decrease in concentration of the dextrin due to the increase in total volume of the dextrin solution during the addition of alcohol results in low yield of dextrin; and the fractionating process highly depends upon the ambient temperature and the repeatability is thus low.
In view of this, the present invention is to provide a novel method for fractionating dextrin and thus to provide a novel method for preparing low-dispersity dextrin. It has been found in the present invention that dextrin is incompatible with polyethylene glycol in aqueous solution and a larger molecular weight of the dextrin results in more significant incompatibility with polyethylene glycol. Accordingly, the stepwise precipitation of dextrin components having a molecular weight from large to small may be realized by gradually increasing the concentration of polyethylene glycol by gradually adding polyethylene glycol to dextrin in aqueous solution. On this basis, the present invention establishes a novel method for fractionating dextrin, which is named “stepwise precipitation method for fractionating dextrin by polyethylene glycol”.